Dire wolf

Dire wolf
Temporal range: Late Pleistocene – early Holocene (125,000–9,000 years ago)
Mounted skeleton, Sternberg Museum of Natural History
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Class: Mammalia
Order: Carnivora
Suborder: Caniformia
Family: Canidae
Genus: Canis
Species: †C. dirus
Binomial name
Canis dirus
Leidy, 1858[1]
Subspecies
Synonyms

The dire wolf (Canis dirus, "fearsome dog") is an extinct species of the genus Canis. It is one of the most famous prehistoric carnivores in North America, along with its extinct competitor, the sabre-toothed cat Smilodon fatalis. The dire wolf lived in the Americas during the Late Pleistocene epoch (125,000–10,000 years ago). The species was named in 1858, four years after the first specimen had been found. Two subspecies are recognized, these being Canis dirus guildayi and Canis dirus dirus. The dire wolf probably evolved from Armbruster's wolf (Canis armbrusteri) in North America. The largest collection of its fossils has been obtained from the Rancho La Brea Tar Pits in Los Angeles.

Dire wolf remains have been found across a broad range of habitats including the plains, grasslands, and some forested mountain areas of North America, and in the arid savannah of South America. The sites range in elevation from sea level to 2,255 metres (7,400 ft). Dire wolf fossils have rarely been found north of 42°N latitude, with five unconfirmed reports above this latitude. This range restriction is thought to be due to temperature, prey, or habitat limitations imposed by proximity to the Laurentide and Cordilleran ice sheets that existed at the time.

The dire wolf was about the same size as the largest modern gray wolves (Canis lupus), which are the Yukon wolf and the northwestern wolf. C. d. guildayi weighed on average 60 kilograms (132 lb) and C. d. dirus was on average 68 kg (150 lb). Its skull and dentition matched those of C. lupus, but its teeth were larger with greater shearing ability, and its bite force at the canine tooth was the strongest of any known Canis species. These characteristics are thought to be adaptations for preying on Late Pleistocene megaherbivores, and in North America its prey are known to have included horses, ground sloths, mastodons, bison, and camels. As with other large Canis hypercarnivores today, the dire wolf is thought to have been a pack hunter. Its extinction occurred during the Quaternary extinction event along with most of the American megafauna of the time, including a number of other carnivores,[note 1] that occurred soon after the appearance of humans in the New World. Its reliance on megaherbivores has been proposed as the cause of its extinction, along with climate change and competition with other species, but the cause remains controversial. Dire wolves lived as recently as 9,440 years ago, according to dated remains.

Taxonomy

Display at the Page Museum of 404 dire wolf skulls found in the La Brea Tar Pits[9]

From the 1850s, the fossil remains of extinct large wolves were being found in the United States, and it was not immediately clear that these all belonged to one species. The first specimen of what would later become associated with Canis dirus was found in mid-1854 in the bed of the Ohio River near Evansville, Indiana. The fossilized jawbone with cheek-teeth was obtained by the geologist Joseph Granville Norwood from an Evansville collector, Francis A. Linck. The paleontologist Joseph Leidy determined that the specimen represented an extinct species of wolf and reported it under the name of Canis primaevus.[3] Norwood's letters to Leidy are preserved along with the type specimen (the first of a species that has a written description) at the Academy of Natural Sciences of Philadelphia. In 1857, while exploring the Niobrara River valley in Nebraska, Leidy found the vertebrae of an extinct Canis species that he reported the following year under the name C. dirus.[1] The name C. primaevus (Leidy 1854) was later renamed Canis indianensis (Leidy 1869) when Leidy found out that the name C. primaevus had previously been used by the British naturalist Brian Houghton Hodgson for the dhole.[4]

In 1876 the zoologist Joel Asaph Allen discovered the remains of Canis mississippiensis (Allen 1876) and associated these with C. dirus (Leidy 1858) and Canis indianensis (Leidy 1869). As so little was found of these three specimens, Allen thought it best to leave each specimen listed under its provisional name until more material could be found to reveal their relationship.[5] In 1908 the paleontologist John Campbell Merriam began retrieving numerous fossilized bone fragments of a large wolf from the Rancho La Brea tar pits. By 1912 he had found a skeleton sufficiently complete to be able to formally recognize these and the previously found specimens under the name C. dirus (Leidy 1858). Because the rules of nomenclature stipulated that the name of a species should be the oldest name ever applied to it,[10] Merriam therefore selected the name of Leidy's 1858 specimen, C. dirus.[11] In 1915 the paleontologist Edward Troxell indicated his agreement with Merriam when he declared C. indianensis a synonym of C. dirus.[12] In 1918, after studying these fossils, Merriam proposed consolidating their names under the separate genus Aenocyon (from Aenos, terrible and cyon, wolf) to become Aenocyon dirus,[7] but not everyone agreed with this extinct wolf being placed in a new genus separate from the genus Canis.[13] Canis ayersi (Sellards 1916) and Aenocyon dirus (Merriam 1918) were recognized as synonyms of C. dirus by the paleontologist Ernest Lundelius in 1972.[14] All of the above taxa were declared synonyms of C. dirus in 1979, according to the paleontologist Ronald M. Nowak.[15]

In 1984 a study by Björn Kurten recognized a geographic variation within the dire wolf populations and proposed two subspecies: Canis dirus guildayi (named by Kurten in honor of the paleontologist John E. Guilday) for specimens from California and Mexico that exhibited shorter limbs and longer teeth, and Canis dirus dirus for specimens east of the North American Continental Divide that exhibited longer limbs and shorter teeth.[2][16][17][18] Kurten designated a maxilla found in Hermit's Cave, New Mexico as representing the nominate subspecies C. d. dirus.[2]

Evolution

Dire wolf divergence

†Canis chihliensis

Canis lupus




†Canis armbrusteri

†Canis gezi


†Canis nehringi


†Canis dirus


†Canis dirus guildayi


†Canis dirus dirus






Phylogeny of Canis dirus based on morphology[19]:148 [20]:181

Both the canid family and its genus Canis are thought to have originated in North America, roughly 40 and 6 million years ago, respectively.[19][21] Members of both radiations subsequently emigrated to the Old World. Wolf-like species of Canis apparently originated there, and then returned to North America.[19]:p148

In 1974 Robert A. Martin proposed that the large North American wolf C. armbrusteri (Armbruster's wolf) was C. lupus.[22] Nowak, Kurten, and Annalisa Berta proposed that C. dirus was not derived from C. lupus.[15][23][24] In 1987, a new hypothesis proposed that a mammal population could give rise to a larger form called a hypermorph during times when food was abundant, but when food later became scarce the hypermorph would either adapt to a smaller form or go extinct. This hypothesis might explain the large body sizes found in many Late Pleistocene mammals compared to their modern counterparts. Both extinction and speciation – a process by which a new species splits from an older one – could occur together during periods of climatic extremes.[25][26] Gloria D. Goulet agreed with Martin, proposing further that this hypothesis might explain the sudden appearance of C. dirus in North America and, judging from the similarities in their skull shapes, that C. lupus had given rise to the C. dirus hypermorph due to an abundance of game, a stable environment, and large competitors.[27]

The three paleontologists Xiaoming Wang, Richard H. Tedford, and Ronald M. Nowak have proposed that C. dirus evolved from Canis armbrusteri,[19]:52[20]:181 with Nowak stating that specimens found in Cumberland Cave, Maryland, appear to be C. armbrusteri diverging into C. dirus.[28][29]:243 The early wolf from China, Canis chihliensis, may have been the ancestor of both C. armbrusteri and the gray wolf C. lupus.[20]:146 The sudden appearance of C. armbrusteri in North America during the Early Pleistocene suggests that this was an immigrant from Asia,[20]:144 as was C. lupus later in the Pleistocene.[20]:144[29]:242 In 2010 Francisco Prevosti proposed that C. dirus was a sister taxon to C. lupus.[30]:472

Canis dirus lived in the late Pleistocene to the early Holocene (125,000–10,000 years before present or YBP) in North and South America (although no members of Canis survive in the latter continent).[2] The majority of fossils from the eastern C. d. dirus have been dated 125,000–75,000 YBP, but the western C. d. guildayi fossils are not only smaller in size but more recent; thus it has been proposed that C. d. guildayi derived from C. d. dirus.[2][18] However, there are disputed specimens of C. dirus that date to 250,000 YBP. Fossil specimens of C. dirus discovered at four sites in the Hay Springs area of Sheridan County, Nebraska, were named Aenocyon dirus nebrascensis (Frick 1930, undescribed), but Frick did not publish a description of them. Nowak later referred to this material as C. armbrusteri;[15]:93 then, in 2009, Tedford formally published a description of the specimens and noted that, although they exhibited some morphological characteristics of both C. armbrusteri and C. dirus, he referred to them only as C. dirus.[20]:146 These Nebraskan fossil specimens may represent the earliest record of C. dirus.[8][20]:146

A fossil discovered in the Horse Room of the Salamander Cave in the Black Hills of South Dakota may possibly be C. dirus; if so, this fossil is one of the earliest specimens on record.[17][31] It was catalogued as Canis cf. C. dirus[32] (where cf. in Latin means confer, uncertain). The fossil of a horse found in the Horse Room provided a uranium-series dating of 252,000 years YBP and the Canis cf. dirus specimen was assumed to be from the same period.[17][32] C. armbrusteri and C. dirus share some characteristics (synapomorphies) that imply the latter's descent from the former. The fossil record suggests C. dirus originated around 250,000 YBP in the open terrain of the mid-continent before expanding eastward and displacing its ancestor C. armbrusteri.[20]:181 The first appearance of C. dirus would therefore be 250,000 YBP in California and Nebraska, and later in the rest of the United States, Canada, Mexico, Venezuela, Ecuador, Bolivia, and Peru,[20]:146 but the identity of these earliest fossils is not confirmed.[33]

In South America, C. dirus specimens dated to the Late Pleistocene were found along the north and west coasts, but none have been found in Argentina, an area that was inhabited by Canis gezi and Canis nehringi.[20]:148 One study found that C. dirus was the most evolutionarily derived Canis species in the New World, and compared to C. nehringi was larger in the size and construction of its lower molars for more efficient predation.[23]:113 For this reason, some researchers have proposed that C. dirus may have originated in South America.[15]:116[17][24] In 2009 a proposal was made that C. armbrusteri was the common ancestor for both the North and South American wolves.[20]:148 The following year, a study yielded evidence that led to the conclusion that C. dirus and C. nehringi were the same species and thus that C. dirus had migrated from North America into South America, making it a late participant in the Great American Interchange.[30]:472

DNA analysis

In 1992 an attempt was made to extract a mitochondrial DNA sequence from the skeletal remains of C. d. guildayi to compare its relationship to other Canis species. The attempt was unsuccessful because these remains had been removed from the La Brea pits and tar could not be removed from the bone material.[34] In 2014 an attempt to extract DNA from a Columbian mammoth from the tar pits also failed, with the study concluding that organic compounds from the asphalt permeate the bones of all ancient samples from the La Brea pits, hindering the extraction of DNA samples.[35]

Radiocarbon dating

The age of most dire wolf localities is determined solely by biostratigraphy, but biostratigraphy is an unreliable indicator within asphalt deposits.[36][37] Some sites have been radiocarbon dated, with C. dirus specimens from La Brea pits dated in calendar years as follows: 82 specimens dated 13,000–14,000 YBP; 40 specimens dated 14,000–16,000 YBP; 77 specimens dated 14,000–18,000 YBP; 37 specimens dated 17,000–18,000 YBP; 26 specimens dated 21,000–30,000 YBP; 40 specimens dated 25,000–28,000 YBP; and 6 specimens dated 32,000–37,000 YBP.[33]:T1 A specimen from Powder Mill Creek Cave, Missouri, was dated at 13,170 YBP.[17]

Description

Dire wolf coming up to human waist height, about as long as a human is in height
Dire wolf size compared to a human

The dire wolf was the largest species of the genus Canis known to have existed.[11][19]:52 Its shape and proportions were similar to those of two modern North American wolves: the Yukon wolf (Canis lupus pambasileus)[20][11] and the Northwestern wolf (Canis lupus occidentalis).[20] The largest northern wolves today have a shoulder height of 38 in (97 cm) and a body length of 69 in (180 cm).[38]:1 Some dire wolf specimens from Rancho La Brea are smaller than this, and some are larger.[11]

The dire wolf had smaller feet and a larger head when compared with a northern wolf of the same body size. The skull length could reach up to 310 mm (12 in) or longer, with a broader palate, frontal region, and zygomatic arches compared with the Yukon wolf. These dimensions make the skull very massive. Its sagittal crest was higher, with the inion showing a significant backward projection, and with the rear ends of the nasal bones extending relatively far back into the skull. A connected skeleton of a dire wolf from Rancho La Brea is difficult to find because the tar allows the bones to disassemble in many directions. Parts of a vertebral column have been assembled, and it was found to be similar to that of the modern wolf, with the same number of vertebrae.[11]

Geographic differences in dire wolves were not detected until 1984, when a study of skeletal remains showed differences in a few cranio-dental features and limb proportions between specimens from California and Mexico (C. d. guildayi) and those found from the east of the Continental Divide (C. d. dirus). A comparison of limb size shows that the rear limbs of C. d. guildayi were 8% shorter than the Yukon wolf due to a significantly shorter tibia and metatarsus, and that the front limbs were also shorter due to their slightly shorter lower bones.[39][40] With its comparatively lighter and smaller limbs and massive head, C. d. guildayi was not as well adapted for running as timber wolves and coyotes.[40][11] C. d. dirus possessed significantly longer limbs than C. d. guildayi. The forelimbs were 14% longer than C. d. guildayi due to 10% longer humeri, 15% longer radii, and 15% longer metacarpals. The rear limbs were 10% longer than C. d. guildayi due to 10% longer femora and tibiae, and 15% longer metatarsals. C. d. dirus is comparable to the Yukon wolf in limb length.[39] The largest C. d. dirus femur was found in Carroll Cave, Missouri, and measured 278 mm (10.9 in).[18]

Skeletons look identical
Gray wolf skeleton (left) compared with a dire wolf skeleton
Canis dirus guildayi compared with the Yukon wolf by the mean length of limb bones in millimeters (inches)
Limb variable C. d. guildayi[40] Yukon wolf[40] C. d. dirus[39]
Humerus (upper front leg) 218 mm (8.6 in) 237 mm (9.3 in) 240 mm (9.4 in)
Radius (lower front leg) 209 mm (8.2 in) 232 mm (9.1 in) 240 mm (9.4 in)
Metacarpal (front foot) 88 mm (3.4 in) 101 mm (4.0 in) 101 mm (4.0 in)
Femur (upper back leg) 242 mm (9.5 in) 251 mm (9.9 in) 266 mm (10.5 in)
Tibia (lower back leg) 232 mm (9.1 in) 258 mm (10.2 in) 255 mm (10.0 in)
Metatarsal (back foot) 93 mm (3.7 in) 109 mm (4.3 in) 107 mm (4.2 in)

C. d. guildayi is estimated to have weighed on average 60 kg (130 lb), and C. d. dirus weighed on average 68 kg (150 lb) with some specimens being larger,[18] but these could not have exceeded 110 kg (240 lb) due to skeletal limits.[41] In comparison, the average weight of the Yukon wolf is 43 kg (95 lb) for males and 37 kg (82 lb) for females. Individual weights for Yukon wolves can vary from 21 kg (46 lb) to 55 kg (121 lb),[42] with one Yukon wolf weighing 79.4 kg (175 lb).[38]:1 These figures show the average dire wolf to be a similar size as the largest modern gray wolf.[18]

The remains of a complete male C. dirus are sometimes easy to identify compared to other Canis specimens because the baculum (penis bone) of the dire wolf is very different from that of all other living canids.[17][39]

Adaptation

Painting of five dire wolves
Restoration of a pack in Rancho La Brea by Charles R. Knight, 1922[43]

Adaptation is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats.[44] Ecological factors such as habitat type, climate, prey specialization, and predatory competition have been shown to greatly influence gray wolf craniodental plasticity, which is an adaptation of the cranium and teeth due to the influences of the environment.[45][46][47] Similarly, the dire wolf was a hypercarnivore, with a skull and dentition adapted for hunting large and struggling prey;[48][49][50] the shape of its skull and snout changed across time, and changes in the size of its body have been correlated to climate fluctuations.[51]

Paleoecology

The last glacial period, commonly referred to as the "Ice Age", spanned 125,000[52]–14,500 YBP[53] and was the most recent glacial period within the current ice age, which occurred during the last years of the Pleistocene era.[52] The Ice Age reached its peak during the Last Glacial Maximum, when ice sheets began advancing from 33,000 YBP and reached their maximum limits 26,500 YBP. Deglaciation commenced in the Northern Hemisphere approximately 19,000 YBP and in Antarctica approximately 14,500 years YBP, which is consistent with evidence that glacial meltwater was the primary source for an abrupt rise in sea level 14,500 YBP.[53] Access into northern North America was blocked by the Wisconsin glaciation. The fossil evidence from the Americas points to the extinction mainly of large animals, termed Pleistocene megafauna, near the end of the last glaciation.[54]

Coastal southern California from 60,000 YBP to the end of the Last Glacial Maximum was cooler and with a more balanced supply of moisture than today. During the Last Glacial Maximum, the mean annual temperature decreased from 11 °C (52 °F) down to 5 °C (41 °F) degrees, and annual precipitation had decreased from 100 cm (39 in) down to 45 cm (18 in).[55] This region was unaffected by the climatic effects of the Wisconsin glaciation, and is thought to have been an Ice Age refugium for animals and cold-sensitive plants.[56][57][58] By 24,000 YBP, the abundance of oak and chaparral decreased, but pines increased, creating open parklands similar to today's coastal montane/juniper woodlands. After 14,000 YBP, the abundance of conifers decreased, and those of the modern coastal plant communities, including oak woodland, chaparral, and coastal sage scrub, increased. The Santa Monica Plain lies north of the city of Santa Monica and extends along the southern base of the Santa Monica Mountains, and 28,000–26,000 YBP it was dominated by coastal sage scrub, with cypress and pines at higher elevations. The Santa Monica Mountains supported a chaparral community on its slopes and isolated coast redwood and dogwood in its protected canyons, along with river communities that included willow, red cedar, and sycamore. These plant communities suggest a winter rainfall similar to that of modern coastal southern California, but the presence of coast redwood now found 600 kilometres (370 mi) to the north indicates a cooler, moister, and less seasonal climate than today. This environment supported large herbivores that were prey for dire wolves and their competitors.[55]

Prey

Sketch
Two dire wolves and a saber-toothed cat (Smilodon) with the carcass of a Columbian mammoth at the La Brea tar pits by R. Bruce Horsfall[59]

A range of animal and plant specimens that became entrapped and were then preserved in tar pits have been removed and studied so that researchers can learn about the past. The Rancho La Brea tar pits located near Los Angeles in southern California are a collection of pits of sticky asphalt deposits that differ in deposition time from 40,000 to 12,000 YBP. Commencing 40,000 YBP, trapped asphalt has been moved through fissures to the surface by methane pressure, forming seeps that can cover several square meters and be 9–11 m (30–36 ft) deep.[36] The dire wolf has been made famous because of the large number of its fossils recovered there.[19]:149 Over 200,000 specimens (mostly fragments) have been recovered from the tar pits,[18] with the remains ranging from Smilodon to squirrels, invertebrates, and plants.[36] The time period represented in the pits includes the Last Glacial Maximum when global temperatures were 8 °C (14 °F) lower than today, the Pleistocene–Holocene transition (Bølling-Allerød interval), the Oldest Dryas cooling, the Younger Dryas cooling from 12,800 to 11,500 YBP, and the American megafaunal extinction event 12,700 YBP when 90 genera of mammals weighing over 44 kg (97 lb) became extinct.[37][51]

Isotope analysis can be used to identify some chemical elements, allowing researchers to make inferences about the diet of the species found in the pits. An isotope analysis of bone collagen extracted from La Brea specimens provides evidence that the dire wolf, Smilodon, and the American lion (Panthera leo atrox) competed for the same prey. Their prey included "yesterday's camel" (Camelops hesternus), the Pleistocene bison (Bison antiquus), the "dwarf" pronghorn (Capromeryx minor), the western horse (Equus occidentalis), and the "grazing" ground sloth (Paramylodon harlani) native to North American grasslands. The Columbian mammoth (Mammuthus columbi) and the American mastodon (Mammut americanum) were rare at La Brea. The horses remained mixed feeders and the pronghorns mixed browsers, but at the Last Glacial Maximum and its associated shift in vegetation the camels and bison were forced to rely more heavily on conifers.[55]

A study of isotope data of La Brea dire wolf fossils dated 10,000 YBP provides evidence that the horse was an important prey species at the time, and that sloth, mastodon, bison, and camel were less common in the dire wolf diet.[48][56] This indicates that the dire wolf was not a prey specialist, and at the close of the late Pleistocene before its extinction it was hunting or scavenging the most available herbivores.[56]

Dentition and bite force

Incisors at the front, followed by canines, followed by premolars, followed by molars at the back
Key features of a wolf skull and dentition

A study of Canis dentition concluded that the dire wolf was the most advanced, or evolutionary derived, Canis species in the Americas. The dire wolf could be identified separately from all other Canis species by its possession of: "P2 with a posterior cusplet; P3 with two posterior cusplets; M1 with a mestascylid, entocristed, entoconulid, and a transverse crest extending from the metaconid to the hyperconular shelf; M2 with entocristed and entoconulid."[23]:50

A study of the estimated bite force at the canine teeth of a large sample of living and fossil mammalian predators, when adjusted for the body mass, found that for placental mammals the bite force at the canines (in newtons/kilogram of body weight) was greatest in the dire wolf (163), followed among the modern canids by the four hypercarnivores that often prey on animals larger than themselves: the African hunting dog (142), the gray wolf (136), the dhole (112), and the dingo (108). The bite force at the carnassials showed a similar trend to the canines. A predator's largest prey size is strongly influenced by its biomechanical limits. The morphology of the dire wolf was similar to that of its living relatives, and assuming that the dire wolf was a social hunter, then its high bite force relative to living canids suggests that it preyed on relatively large animals. The bite force rating of the bone-consuming spotted hyena (117) challenged the common assumption that high bite force in the canines and the carnassials was necessary to consume bone.[50]

A study of the cranial measurements and jaw muscles of dire wolves found no significant differences with modern gray wolves in all but 4 of 15 measures. Upper dentition was the same except that the dire wolf had larger dimensions, and the P4 had a relatively larger, more massive blade that enhanced slicing ability at the carnassial. The jaw of the dire wolf had a relatively broader and more massive temporalis muscle, able to generate slightly more bite force than the gray wolf. Due to the jaw arrangement, the dire wolf had less temporalis leverage than the gray wolf at the lower carnassial (m1) and lower p4, but the functional significance of this is not known. The lower premolars were relatively slightly larger than those of the gray wolf,[49] and the dire wolf m1 was much larger and had more shearing ability.[11][24][49] The dire wolf canines had greater bending strength than those of living canids of equivalent size and were similar to those of hyenas and felids.[60] All these differences indicate that the dire wolf was able to deliver stronger bites than the gray wolf, and with its flexible and more rounded canines was better adapted for struggling with its prey.[48][49]

Three views of the skull from the side, above, and underneath
Skull of the dire wolf[61]
Canis lupus and Canis dirus compared by mean mandible tooth measurements (millimeters)
Tooth variable lupus modern

North American[62]

 lupus La Brea[62] lupus Beringia[62] dirus dirus Sangamon era[2][48]

(125,000–75,000 YBP)

 dirus dirus Late Wisconsin[2][48]

(50,000 YBP)

 dirus guildayi[2][48]

(40,000–13,000 YBP)

m1 length 28.2 28.9 29.6 36.1 35.2 33.3
m1 width 10.7 11.3 11.1 14.1 13.4 13.3
m1 trigonid length 19.6 21.9 20.9 24.5 24.0 24.4
p4 length 15.4 16.6 16.5 16.7 16.0 19.9
p4 width - - - 10.1 9.6 10.3
p2 length - - - 15.7 14.8 15.7
p2 width - - - 7.1 6.7 7.4

Behavior

At La Brea, predatory birds and mammals were attracted to dead or dying herbivores that had become mired, and then these predators became trapped themselves.[36][63] Herbivore entrapment was estimated to have occurred once every fifty years,[63] and for every instance of herbivore remains found in the pits there were an estimated ten carnivores.[36] C. d. guildayi is the most common carnivoran found at La Brea, followed by Smilodon.[37][51] Remains of dire wolves outnumber remains of gray wolves in the tar pits by a ratio of five to one.[33] During the Last Glacial Maximum, coastal California, with a climate slightly cooler and wetter than today, is thought to have been a refuge,[56] and a comparison of the frequency of dire wolves and other predator remains at La Brea to other parts of California and North America indicates significantly greater abundances; therefore, the higher dire wolf numbers in the La Brea region did not reflect the wider area.[64] Assuming that only a few of the carnivores that were feeding became trapped, it is likely that fairly sizeable groups of dire wolves fed together on these occasions.[65]

Baculum bone is very long
Skeleton from the La Brea Tar Pits mounted in running pose. Note the baculum between the rear legs.

The difference between the male and female of a species apart from their sex organs is called sexual dimorphism, and in this regard little variance exists among the canids. A study of dire wolf remains dated 15,360–14,310 YBP and taken from one pit that focused on skull length, canine tooth size, and lower molar length showed little dimorphism, similar to that of the gray wolf, indicating that dire wolves lived in monogamous pairs.[65] Their large size and highly carnivorous dentition supports the proposal that the dire wolf was a predator that fed on large prey.[65][66][67] To kill ungulates larger than themselves, the African wild dog, the dhole, and the gray wolf depend on their jaws as they cannot use their forelimbs to grapple with prey, and they work together as a pack consisting of an alpha pair and their offspring from the current and previous years. It can be assumed that dire wolves lived in packs of relatives that were led by an alpha pair.[65] Large and social carnivores would have been successful at defending carcasses of prey trapped in the tar pits from smaller solitary predators, and thus the most likely to become trapped themselves. The many C. d. guildayi and Smilodon remains found in the tar pits suggests that both were social predators.[64][68]

All social terrestrial mammalian predators prey mostly on terrestrial herbivorous mammals with a body mass similar to the combined mass of the social group members attacking the prey animal.[41][69] The large size of the dire wolf provides an estimated prey size in the 300 to 600 kg (660 to 1,320 lb) range.[18][66][67] Stable isotope analysis of dire wolf bones provides evidence that they had a preference for consuming ruminants such as bison rather than other herbivores but moved to other prey when food became scarce, and occasionally scavenged on beached whales along the Pacific coast when available.[18][49][70] A pack of timber wolves can bring down a 500 kg (1,100 lb) moose that is their preferred prey,[18][38]:76 and a pack of dire wolves bringing down a bison is conceivable.[18] Although some studies have suggested that because of tooth breakage, the dire wolf must have gnawed bones and may have been a scavenger, its widespread occurrence and the more gracile limbs of C. d. dirus indicate a predator. Like the gray wolf today, the dire wolf probably used its post-carnassial molars to gain access to marrow, but the dire wolf's larger size enabled it to crack larger bones.[49]

Tooth breakage

The teeth are very large in comparison to the modern wolf.
Dire wolf skull and neck
Nibbling by the incisors at the front of the mouth, next the canines for seizing, next the premolars for chewing, next the carnassials and molars for cutting and cracking
Dentition of an Ice Age wolf

Tooth breakage is related to a carnivore's behavior.[71] A study of nine modern carnivores found that one in four adults had suffered tooth breakage and that half of these breakages were of the canine teeth. The most breakage occurred in the spotted hyena that consumes all of its prey including the bone; the least breakage occurred in the African wild dog, and the gray wolf ranked in between these two.[72][71] The eating of bone increases the risk of accidental fracture due to the relatively high, unpredictable stresses that it creates. The most commonly broken teeth are the canines, followed by the premolars, carnassial molars, and incisors. Canines are the teeth most likely to break because of their shape and function, which subjects them to bending stresses that are unpredictable in both direction and magnitude. The risk of tooth fracture is also higher when killing large prey.[72]

A study of the fossil remains of large carnivores from La Brea pits dated 36,000–10,000 YBP shows tooth breakage rates of 5–17% for the dire wolf, coyote, American lion, and Smilodon, compared to 0.5–2.7% for ten modern predators. These higher fracture rates were across all teeth, but the fracture rates for the canine teeth were the same as in modern carnivores. The dire wolf broke its incisors more often when compared to the modern gray wolf; thus, it has been proposed that the dire wolf used its incisors more closely to the bone when feeding. Dire wolf fossils from Mexico and Peru show a similar pattern of breakage. A 1993 study proposed that the higher frequency of tooth breakage among Pleistocene carnivores compared with living carnivores was not the result of hunting larger game, something that might be assumed from the larger size of the former. When there is low prey availability, the competition between carnivores increases, causing them to eat faster and thus consume more bone, leading to tooth breakage.[51][71][73] As their prey became extinct around 10,000 years ago, so did these Pleistocene carnivores, except for the coyote (which is an omnivore).[71][73]

A later La Brea pits study compared tooth breakage of dire wolves in two time periods. One pit contained fossil dire wolves dated 15,000 YBP and another dated 13,000 YBP. The results showed that the 15,000 YBP dire wolves had three times more tooth breakage than the 13,000 YBP dire wolves, whose breakage matched those of nine modern carnivores. The study concluded that between 15,000–14,000 YBP prey availability was less or competition was higher for dire wolves, and that by 13,000 YBP, as the prey species moved towards extinction, predator competition had declined and therefore the frequency of tooth breakage in dire wolves had also declined.[73][74]

Carnivores include both pack hunters and solitary hunters. The solitary hunter depends on a powerful bite at the canine teeth to subdue their prey, and thus exhibits a strong mandibular symphysis. In contrast, a pack hunter, which delivers many shallower bites, has a comparably weaker mandibular symphysis. Thus, researchers can use the strength of the mandibular symphysis in fossil carnivore specimens to determine what kind of hunter it was – a pack hunter or a solitary hunter – and even how it consumed its prey. The mandibles of canids are buttressed behind the carnassial teeth to enable the animals to crack bones with their post-carnassial teeth (molars M2 and M3). A study found that the mandible buttress profile of the dire wolf was lower than that of the gray wolf and the red wolf, but very similar to the coyote and the African hunting dog. The dorsoventrally weak symphyseal region (in comparison to premolars P3 and P4) of the dire wolf indicates that it delivered shallow bites similar to its modern relatives and was therefore a pack hunter. This suggests that the dire wolf may have processed bone but was not as well adapted for it as was the gray wolf.[75] The fact that the incidence of fracture for the dire wolf reduced in frequency in the late Pleistocene to that of its modern relatives[71][74] suggests that reduced competition had allowed the dire wolf to return to a feeding behaviour involving a lower amount of bone consumption, a behaviour for which it was best suited.[73][75]

The results of a study of dental microwear on tooth enamel for specimens of the carnivore species from La Brea pits, including dire wolves, suggest that these carnivores were not food-stressed just before their extinction. The evidence also indicated that the extent of carcass utilization (i.e., amount consumed relative to the maximum amount possible to consume, including breakup and consumption of bones) was less than among large carnivores today. These finding indicates that tooth breakage was related to hunting behavior and the size of prey.[76]

Climate impact

Past studies proposed that changes in dire wolf body size correlated with climate fluctuations.[51][77] A later study compared dire wolf craniodental morphology from four La Brea pits, each representing four different time periods. The results are evidence of a change in dire wolf size, dental wear and breakage, skull shape, and snout shape across time. Dire wolf body size had decreased between the start of the Last Glacial Maximum and near its ending at the warm Allerød oscillation. Evidence of food stress (food scarcity leading to lower nutrient intake) is seen in smaller body size, skulls with a larger cranial base and shorter snout (shape neoteny and size neoteny), and more tooth breakage and wear. Dire wolves dated 17,900 YBP showed all of these features, which indicates food stress. Dire wolves dated 28,000 YBP also showed to a degree many of these features but were the largest wolves studied, and it was proposed that these wolves were also suffering from food stress and that wolves earlier than this date were even bigger in size.[51] Nutrient stress is likely to lead to stronger bite forces to more fully consume carcasses and to crack bones,[51][78] and with changes to skull shape to improve mechanical advantage. North American climate records reveal cyclic fluctuations during the glacial period that included rapid warming followed by gradual cooling, called Dansgaard–Oeschger events. These cycles would have caused increased temperature and aridity, and at La Brea would have caused ecological stress and therefore food stress.[51] A similar trend was found with the gray wolf, which in the Santa Barbara basin was originally massive, robust, and possibly convergent with the dire wolf, but was replaced by more gracile forms by the start of the Holocene.[15][27][51]

Canis dirus information based on skull measurements[51]
Variable 28,000 YBP 26,100 YBP 17,900 YBP 13,800 YBP
Body size largest large smallest medium/small
Tooth breakage high low high low
Tooth wear high low high low
Snout shape shortening, largest cranial base average shortest, largest cranial base average
Tooth row shape robust - - gracile
DO event number 3 or 4 none imprecise data imprecise data

Competitors

Mounted skeletons of Smilodon and dire wolf near ground sloth bones

Just before the appearance of the dire wolf, North America was invaded by the genus Xenocyon (ancestor of the Asian dhole and the African hunting dog) that was as large as the dire wolf and more hypercarnivorous. The fossil record shows them as rare, and it is assumed that they could not compete with the newly derived dire wolf.[19]:60 Stable isotope analysis provides evidence that the dire wolf, Smilodon, and the American lion competed for the same prey.[55][76] Other large carnivores included the extinct North American giant short-faced bear (Arctodus simus), the modern cougar (Puma concolor), the Pleistocene coyote (Canis latrans), and the Pleistocene gray wolf that was more massive and robust than today. These predators may have competed with humans who hunted for similar prey.[76]

Specimens that have been identified by morphology as Beringian wolves (C. lupus) and radiocarbon dated 25,800–14,300 YBP have been found in the Natural Trap Cave at the base of the Bighorn Mountains in Wyoming, in the western United States. The location is directly south of what would at that time have been a division between the Laurentide Ice Sheet and the Cordilleran Ice Sheet. A temporary channel between the glaciers may have existed that allowed these large, Alaskan direct competitors of the dire wolf, which were also adapted for preying on megafauna, to come south of the ice sheets. Dire wolves were absent north of 42°N latitude in the Late Pleistocene; therefore, this region would have been available for Beringian wolves to expand south along the glacier line. How widely they were then distributed is not known. These also became extinct at the end of the Late Pleistocene, as did the dire wolf.[33]

Range

Map of US states shaded gray where Canis dirus remains have been found

Dire wolf remains have been found across a broad range of habitats including the plains, grasslands, and some forested mountain areas of North America, and in the arid savannah of South America. The sites range in elevation from sea level to 2,255 m (7,400 ft).[17] The location of these fossil remains suggests that dire wolves lived predominantly in the open lowlands along with its prey the large herbivores.[15] Dire wolf remains do not occur at high latitudes, unlike its close relative the gray wolf.[17]

In the United States, dire wolf fossils have been reported in Arizona, California, Florida, Idaho, Indiana, Kansas, Kentucky, Missouri, Nebraska, New Mexico, Oregon, Pennsylvania, South Carolina, South Dakota, Texas, Utah, Virginia, West Virginia, Wyoming,[17] and Nevada.[79] The identity of fossils reported farther north than California is not confirmed.[33] There have been five reports of unconfirmed dire wolf fossils north of 42°N latitude at Fossil Lake, Oregon (125,000–10,000 YBP), American Falls Reservoir, Idaho (125,000–75,000 YBP), Salamander Cave, South Dakota (250,000 YBP), and four closely grouped sites in northern Nebraska (250,000 YBP).[33] This suggests a range restriction on dire wolves due to temperature, prey, or habitat.[33] The major fossil-producing sites for C. d. dirus are located east of the Rocky Mountains and include Friesenhahn Cave, near San Antonio, Texas; Carroll Cave, near Richland, Missouri; and Reddick, Florida.[18]

Localities in Mexico where dire wolf remains have been collected include El Cedazo in Aguascalientes, Comondú Municipality in Baja California Sur, El Cedral in San Luis Potosí, El Tajo Quarry near Tequixquiac, state of Mexico, Valsequillo in Puebla, Lago de Chapala in Jalisco, Loltun Cave in Yucatán, Potrecito in Sinaloa, San Josecito Cave near Aramberri in Nuevo León and Térapa in Sonora. The specimens from Térapa were confirmed as C. d. guildayi.[48] The finds at San Josecito Cave and El Cedazo have the greatest number of individuals from a single locality.

In South America, dire wolves have been dated younger than 17,000 YBP and reported from only three localities: Muaco in Falcón state, Venezuela; Talara Province in Peru; and Tarija Department in Bolivia.[17] If the dire wolf originated in North America, the species likely dispersed into South America via the Andean corridor,[17][23] a proposed pathway for temperate mammals to migrate from Central to South America because of the favorable cool, dry, and open habitats that characterized the region at times. This most likely happened during a glacial period because the pathway then consisted of open, arid regions and savanna, whereas during inter-glacial periods it would have consisted of tropical rain forest.[17][80]

Extinction

During the American megafaunal extinction event around 12,700 YBP, 90 genera of mammals weighing over 44 kilograms (97 lb) became extinct.[37][51] The extinction of the large carnivores and scavengers is thought to have been caused by the extinction of the megaherbivore prey upon which they depended.[81][82][17][71] The cause of the extinction of the megafauna is debated[76] but has been attributed to the impact of climate change, competition with other species including overexploitation by newly arrived human hunters, or a combination of both.[76][83] One study proposes that several extinction models should be investigated because so little is known about the biogeography of the dire wolf and its potential competitors and prey, nor how all these species interacted and responded to the environmental changes that occurred at the time of extinction.[17]

Ancient DNA and radiocarbon data indicates that local genetic populations were replaced by others within the same species or by others within the same genus.[84] Both the dire wolf and the Beringian wolf went extinct in North America, leaving only the less carnivorous and more gracile form of the wolf to thrive,[62] which may have outcompeted the dire wolf.[85] Dire wolf remains having the youngest geological ages are dated at 9,440 YBP at Brynjulfson Cave, Boone County, Missouri,[24][85] 9,860 YBP at Rancho La Brea, California, and 10,690 YBP at La Mirada, California.[85] Dire wolf remains have been radiocarbon dated to 8,200 YBP from Whitewater Draw in Arizona,[83][86] However, one author has stated that radiocarbon dating of bone carbonate is known to be unreliable, and that while many dire wolf remains are found in fossil assemblages dated to the terminal Pleistocene, there are no confirmed Holocene finds.[17]

See also

Notes

  1. ↑ Other North American carnivorans that became extinct at about the same time include short-faced bears, American lions, Smilodon fatalis, Homotherium serum, Miracinonyx and dholes. Additionally, North American pumas and jaguars (but not those in South America) apparently were temporarily extirpated.

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